First, put a proper terminal on the cable end with an inline fuse on the power cable, or you're going to burn your car to a crisp.
Ok, with that out of the way... You're probably not getting enough current through the power cable with the way it's currently attached. Is your ground cable the same size as the power feed? Attached to some nice shiny bare metal? Where are you getting the signal from for the RCA's, if you can't find the amps for the Mach system? They should be under the rear deck attached to the speaker enclosure. If that's not there and you don't have tweeters on your doors, next to the mirrors, it's not the Mach system.